My understanding is that the HPF is only on when you engage it with the footswitch which will disable the crossover in the sub and send full bandwidth to your top speakers. When the sub is engaged the crossover is turned back on and your LPF comes into play at that point

Its confusing to me as well

" High Pass Filter - Removes frequency content below 80 Hz from the full-range signal sent from the T10 outputs. The High Pass filter is bypassed when the Sub Bypass is enabled. "